1. Field of the Invention
The present invention relates to a positive electrode active material for alkaline storage batteries used for a positive electrode of an alkaline storage battery such as a nickel-hydrogen storage battery or a nickel-cadmium storage battery, a positive electrode for alkaline storage batteries using such a positive electrode active material for alkaline storage batteries, and an alkaline storage battery, and is particularly characterized in that the positive electrode active material for alkaline storage batteries is improved, to obtain a high discharge capacity in the alkaline storage battery.
2. Description of the Related Art
Conventionally, in an alkaline storage battery such as a nickel-hydrogen storage battery or a nickel-cadmium storage battery, a sintered nickel electrode and a non-sintered nickel electrode have been used as its positive electrode.
In the sintered nickel electrode, a porous sintered nickel substrate obtained by sintering has been used, and the porous sintered nickel substrate has been chemically impregnated with a salt of an active material and has been filled with the active material.
In an alkaline storage battery using such a sintered nickel electrode, it has been necessary to use a sintered nickel substrate having a high degree of porosity to fill the sintered nickel substrate with a lot of active materials in order to obtain a sufficient battery capacity.
When the sintered substrate having a high degree of porosity is used, however, a bond between nickel particles by the sintering is weak. Accordingly, the nickel particles drop out of the sintered substrate. Further, the pore diameter in the sintered nickel substrate is generally small, for example, not more than 10 xcexcm. In order to sufficiently fill the sintered nickel substrate with the active material, therefore, laborious work for impregnating the sintered nickel substrate with the active material must be repeatedly performed again and again, thereby degrading productivity.
In recent years, a non-sintered nickel electrode using a paste obtained by adding a binder such as methyl cellulose to an active material mainly composed of a nickel hydroxide and so adapted as to fill a conductive base material having a high degree of porosity such as foamed nickel with the paste has been used.
In the case of the non-sintered nickel electrode, it is possible to use a conductive base material having a high degree of porosity such as foamed nickel, as described above, to fill the conductive base material with a lot of active materials as well as to make it easy to perform work for filling the conductive base material with the active materials.
When the base material having a high degree of porosity is used, as described above, in the non-sintered nickel electrode, however, collecting current in the base material becomes difficult, thereby reducing the utilization of the active materials. When the non-sintered nickel electrode is used for a positive electrode of the alkaline storage battery, a sufficient battery capacity cannot be obtained.
In recent years, in order to increase the utilization of an active material in a non-sintered nickel electrode, it has been proposed that used as a positive electrode active material for alkaline storage batteries is one obtained by coating a surface of a nickel hydroxide with a cobalt compound indicated by a structural formula Co1xe2x88x92xAx(OH)2By or Co1xe2x88x92xAxOOH [in the structural formula, A is an element belonging to any one of the groups 1B, 2A, 2B, 3A, 4A, 5A, 6A, 7A, and 8A of the periodic system, boron, or aluminum, B is any one of a sulfate ion, a nitrate ion, a carbonate ion, a borate ion, and a phosphate ion, x is 0.05 to 0.5, and y is {(the valence of the element A)xe2x88x922}xc3x97x/(the valence of the ion B).], as disclosed in JP-A-10-21901.
Even when the positive electrode active material described in the above-mentioned gazette is used for the positive electrode of the alkaline storage battery, however, conductivity of the positive electrode active material are not sufficient, thereby making it difficult to sufficiently increase the utilization of the positive electrode active material. Accordingly, a high discharge capacity cannot be still obtained.
An object of the present invention is to improve a positive electrode active material used for a positive electrode of an alkaline storage battery to sufficiently improve conductivity of the positive electrode of the alkaline storage battery and increase the utilization of the positive electrode active material, thereby obtaining a high discharge capacity in the alkaline storage battery.
The present invention is directed to a positive electrode active material for alkaline storage batteries, wherein a surface of a nickel hydroxide is coated with a mixed crystal material of at least one type of element selected from aluminum Al, manganese Mn, iron Fe, yttrium Y, ytterbium Yb, erbium Er, and gadolinium Gd and cobalt, the valence of nickel in the nickel hydroxide being in the range of 2.0 to 2.3, and the valence of cobalt in the mixed crystal material exceeding 3.0.
As in the positive electrode active material for alkaline storage batteries according to the present invention, when the surface of the nickel hydroxide is coated with the above-mentioned mixed crystal material, conductivity of the positive electrode active material are improved, so that the utilization of the positive electrode active material is increased, thereby making it possible to obtain a high discharge capacity.
The reason why in the positive electrode active material for alkaline storage batteries in the present invention, the valence of nickel in the nickel hydroxide is in the range of 2.0 to 2.3, as described above, is that when the valence of Ni exceeds 2.3, a capacity in first charging the alkaline storage battery using the positive electrode active material is reduced and is determined by the capacity of a negative electrode, thereby making it impossible to obtain a high discharge capacity.
The reason why in the positive electrode active material for alkaline storage batteries in the present invention, the valence of cobalt in the mixed crystal material exceeds 3.0, as described above, is that when the valence of cobalt is not more than 3.0, conductivity of the positive electrode active material cannot be sufficiently improved, so that the utilization of the positive electrode active material is reduced, thereby making it impossible to obtain a high discharge capacity.
In setting the valence of nickel in the nickel hydroxide in the range of 2.0 to 2.3 and setting the valence of cobalt in the mixed crystal material to more than 3.0, as described above, the surface of the nickel hydroxide is coated with the mixed crystal material of the above-mentioned element and cobalt, and is then oxidized by an oxidizing agent such as sodium hypochlorite under the presence of an alkaline solution or is oxidized with air under the presence of the alkaline solution. Examples of the oxidizing agent include known oxidizing agents such as peroxodisulfate, persulfate, and hydrogen peroxide in addition to the sodium hypochlorite.
In the mixed crystal material of at least one type of element selected from aluminum Al, manganese Mn, iron Fe, yttrium Y, ytterbium Yb, erbium Er, and gadolinium Gd with which the surface of the nickel hydroxide is coated and cobalt, when the content of Al, Mn, Fe, Y, Yb, Er, and Gd in the mixed crystal material is less than 5 wt % (% by weight) of the total amount of the elements and cobalt, it is difficult for the valence of cobalt in the mixed crystal material to exceed 3.0, thereby making it impossible to sufficiently improve the conductivity of the positive electrode active material. On the other hand, when the content of Al, Mn, Fe, Y, Yb, Er, and Gd in the mixed crystal material is increased to exceed 50 wt %, the conductivity of the mixed crystal material is reduced, thereby making it impossible to sufficiently improve the conductivity of the positive electrode active material. Therefore, it is preferable that the total amount of the elements, i.e., aluminum Al, manganese Mn, iron Fe, yttrium Y, ytterbium Yb, erbium Er, and gadolinium Gd contained in the mixed crystal material is in the range of 5 to 50 wt % of the total amount of the elements and cobalt.
In coating the surface of the nickel hydroxide with the mixed crystal material of at least one type of element selected from aluminum Al, manganese Mn, iron Fe, yttrium Y, ytterbium Yb, erbium Er, and gadolinium Gd and cobalt, as described above, when the amount of the mixed crystal material is small, the conductivity of the positive electrode active material cannot be sufficiently improved. On the other hand, if the amount of the mixed crystal material is too large, the ratio of the nickel hydroxide to be charged or discharged is reduced, thereby making it impossible to obtain a sufficient battery capacity. Therefore, it is preferable that the amount of cobalt in the mixed crystal material with which the surface of the nickel hydroxide is coated is in the range of 1 to 10 wt % of the amount of the nickel hydroxide.